Shabozov, M. S., and M. O. Akobirshoev. “Mean-Square Approximation by an Angle in $L_2$ and the Values of Quasiwidths for Some Classes of Functions”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 72, no. 6, June 2020, pp. 852-64, doi:10.37863/umzh.v72i6.1064.